Remote Connection problem

My machine at the office is Win XP Pro SP 1, and my machine at home is Win XP
Pro SP 1. I went to control panel - System - Remote tab, and check the box
"Allow users to connect remotely to this computer".
For 1 month, I am able to connect from my home machine to my machine at the
office.

But now, out of a sudden, when I try to connect from home to my office, I
got the error:
"The client could not connect to the remote computer.
Remote connections might not be enabled or the computer might be too busy to
accept new connections.
It is also possible that the network problems are preventing your connection.
Please try connecting again later. If the problem continues to occur,
contact your administrator."

I asked somebody from my office to reboot my office's machine, but I still
have a problem after that.
Nobody is using my office computer.
I have no firewall on my office computer.
How can I fix this problem ?

Thanks.